THE MICROBIOLOGY OF THE ATMOSPHERE 



the Advancement of Science's book on ^Aerobiology^ (Moulton, 1942) has 

 no chapter on bacteria in outdoor air over land. 



Recent Study of Fungi and Pollen 



The pollen and fungus components of the outdoor air-spora have 

 attracted much attention in this century, and the development and ex- 

 tensive use of volumetric sampling equipment for the purpose has been 

 highly illuminating. Quantitative visual counting of spores from 3 /x in 

 diameter and upwards, confirms Miquel's impression of recurrent 'tides' 

 of spore concentration ; but different groups of organisms are now known 

 to have separate 'tidal waves', and the concentration and composition of 

 the air-spora varies enormously with place, season, time of day, weather, 

 and human activity. 



THE AIR-SPORA AT 2 METRES ABOVE GROUND-LEVEL 



Continuous records in a mixed agricultural environment were obtained 

 during the summer of 1952 by Gregory & Hirst (1957), using the Hirst 

 automatic volumetric spore-trap. The mean spore concentration at 2 

 metres above the ground over the period i June to 25 October was 12,500 

 spores per cubic metre. These were grouped visually into twenty-five 

 categories. The commonest spore-type was Chidosporiimi (probably mainly 

 C. herbarum)^ which accounted for 47 per cent of the total. The second 

 commonest were classified as hyaline basidiospores and made up 31 per 

 cent of the season's catch; most of these were probably spores of species 

 of Sporobolomyces, with Tilletiopsis adding another 0-56 per cent. Coloured 

 basidiospores of mushrooms and toadstools (agarics, boleti, and bracket 

 fungi) amounted to 3-3 per cent of the season's total. Pollen made up i 

 per cent of the total. Conidia of pow^dery mildews (Erysiphaceae), 'brand 

 spores' of Ustilago species, and conidia of Altemaria, amounted to between 

 I and 2 per cent each. Ten other recognizable categories contributed 

 between 0-03 and 0-5 per cent each. All other particles recognizable as 

 spores of micro-organisms were put in the 'unclassified' category, totalling 

 8 per cent of the season's catch, and included many organisms which, 

 although abundant in soils, form only an insignificant fraction of the 

 summer outdoor air-spora (for example, PeniciUiimi, Aspergillus, and 

 various Mucoraceae). Bacteria and actinomycetes are not revealed by this 

 trap method, which is efficient only for particles over 3 /^ in diameter. 



In Britain the attempts to get a relatively undistorted picture of the 

 outdoor air-spora have demonstrated beyond doubt that Cladosporium 

 and Sporobolomyces predominate, followed by the hyaline and coloured 

 basidiospores of the mushrooms and toadstools. Fewer in number, but not 

 necessarily less in total volume, are the pollens, Alternaria, ascospores, 

 and the large-spored plant-pathogenic fungi. Under ordinary conditions, 

 splash-borne types seem not to amount to more than a few per cent of the 



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